Bi-level switching of fluorescent lamps allows space to be illuminated as needed by providing a high level of illumination when the space is occupied and a lower level of illumination when it is not. This can be accomplished by lighting all of the fluorescent lamps for high level illumination and lighting some of the fluorescent lamps for lower level illumination. As an alternative, the lamps can be run at a reduced power level. Energy use and energy cost will be reduced if lights are switched off or run at a reduced power for lower level illumination. The illumination level can be controlled manually, with timers, or with sensors able to detect when the room is occupied.
Bi-level switching of fluorescent lamps has been accomplished using a triac to switch power at the ballast output, but using a triac does not allow continuous lighting. Such switching is described in U.S. Pat. No. 5,808,423 to Li et al., assigned to the same assignee as the present invention and incorporated herein by reference. The energy savings is accomplished by switching off one or more lamps. The ballast must be toggled off between the high power level of the high level illumination and the low power level of the lower level illumination because the triac remains latched until power is removed completely. This approach is inconvenient to the occupants, since the light is switched off to switch from high level to low level illumination.
U.S. Ser. No. 09/867,261 filed May 29, 2001, assigned to the same assignee as the present invention and incorporated herein by reference, improves bi-level ballast control through the use of an additional lead wire. Toggling of the input voltage is not required, but one or more lamps must still be switched off using a power switch and an optocoupler. Switching is also known to decrease the life of lamps and may decrease the useful life of other lighting system components.
U.S. Pat. No. 6,204,614 to Erhardt, assigned to the same assignee as the present invention and incorporated herein by reference, describes power level switching without the need to switch off lamps, which can even be used in single lamp systems. The system as described uses a ballast with a feedback loop, which can be more complex and costly than an open loop ballast. Open loop ballasts normally operate at a fixed frequency.
It would be desirable to have electronic switching for an open loop bi-level ballast control that would overcome the above disadvantages.